Dynamically fetch event documentation

This is part one to update event documentation dynamically and related to #1873.

The only thing missing is an AJAX page returning the event's documentation based on the given parameters, `section`, `filters` and `name`. The script currently requests that data at `/symphony/ajax/event-documentation/` but that can of course be changed.

Pull request #1888 needs to be pulled first for styling reasons.

/cc @brendo